Mike - I would have thought the major suppliers would have them. Typically called "High Back" I think. I know Moss lists them (fairly pricey), and Spitbits. I assume TRF and VickBrit would.
The highbacks are more money than the ones with detachable headrests, but then once you buy the headrest kits maybe it all works out.
I got a low-back upholstery kit from Canley Classics in the UK last year. Pretty reasonable, although shipping added quite a lot, and good quality. They list their sets for MkIV now at ÂŁ137. I'm not sure if the highback seat was sold in the UK though, so these may be for detachable headrest seats.